1. Field of the Invention
This invention relates to an automatic focus adjusting device for use in a camera or the like.
2. Related Background Art
A method of correcting the out-of-focus condition of the object lens attributable to the movement of an object when pursuing the moving object using AF has already been proposed by the assignee of Japanese Patent Application No. 62-263728.
In the embodiment of the above-mentioned patent application, the movement of the image plane of the object is approximated to a quadratic function or a linear function, while the time required for distance measurement calculation, lens driving or release is foreseen under a certain assumption and the position of the image plane of the object at a certain time in the future (for example, the time when the control of lens driving is completed or the time when the shutter curtains are moved after the release operation) is foreseen, and in accordance with the result thereof, lens driving is effected and the delay in the pursuit for the moving object is eliminated.
FIG. 2 of the accompanying drawings is a graph for illustrating the above-described method of correcting lens driving. In the figure, the horizontal axis represents time t, and the vertical axis represents the position d of the image plane of the object.
A curve f(t) indicated by a solid line represents the position of the image plane of the object at a time t when the object approaches the camera in the direction of the optic axis when the photo-taking lens is at infinity. A curve l(t) indicated by a broken line represents the position of the image plane of the object at the position of the photo-taking lens at the time t, and a section [t.sub.i, t.sub.i '] is the time of the focus detecting operation, and a section [t.sub.i ', t.sub.i+1 ] is the time of the lens driving operation. Accordingly, the difference in the direction of the vertical axis d between f(t) and l(t) at the same time t corresponds to the so-called defocus amount.
DFi represents the defocus amount detected at a time t.sub.i, DLi represents the lens driving amount as converted into the image plane moving amount executed from the result of the focus detection at a time t.sub.i-1, and TM.sub.i represents the time interval between the focus detecting operations.
In the example shown in FIG. 2, the assumption that the position of the image plane of the object changes in accordance with a quadratic function is placed as a premise for correction. That is, it is assumed that if the current and past three positions (t.sub.1, f.sub.1), (t.sub.2, f.sub.2) and (t.sub.3, f.sub.3) of the image plane are known at a time t.sub.3, the position f.sub.4 of the image plane at a time t.sub.4 can be foreseen.
However, what the camera can actually detect are not the positions f.sub.1, f.sub.2 and f.sub.3 of the image plane, but the defocus amounts DF1, DF2 and DF3 and the lens driving amounts DL1 and DL2 as converted into the image plane moving amounts. The time t.sub.4 is a future value, and actually is a value which varies as the accumulating time of an accumulation type sensor is varied by the brightness of the object, but here, for simplicity, it is assumed to be a known value in the relation that t.sub.4 -t.sub.3 =t.sub.3 -t.sub.2.
Under the above-described assumption, the lens driving amount DL3 when lens driving is effected toward t.sub.4 at a time t.sub.3, from the result of the focus detection at the time t.sub.3 is found from the following equations: EQU at.sup.2 +bt+c=f(t) (1) EQU at.sub.1.sup.2 +bt.sub.1 +c=f.sub.1 ( 2) EQU at.sub.2.sup.2 +bt.sub.2 +c=f.sub.2 ( 2)' EQU at.sub.3.sup.2 +bt.sub.3 +c=f.sub.3 ( 2)"
If in FIG. 2, the point l.sub.1 is considered to be the origin, EQU f.sub.1 =DF1,f.sub.2 =DF2+DF1,f.sub.3 =DF3+DL2+DL1 (3) EQU t.sub.1 0,t.sub.2 =TM1,t.sub.3 =TM1+TM2 (4)
If the equations (3) and (4) are substituted into the equations (2), (2)' and (2)", a, b and c are found as follows: ##EQU1## Consequently, the lens driving amount DL3 as converted into the image plane moving amount at the time t.sub.4 is ##EQU2##
Here, assuming that TM3 is known in the relation that TM3=TM2 as previously described, DL3 is found from the equation (8).
In the same manner, the lens driving amounts at the time t.sub.4 and subsequent time t.sub.n can be found from the past three detected defocus amounts DF.sub.n-2' DF.sub.n-1 and DF.sub.n, the past two actual lens driving amounts DL.sub.n-2 and DL.sub.n-1 and the past two time intervals TM.sub.n-2 and TM.sub.n-1. ##EQU3## If in accordance with the equations (8), (9) and (10), the defocus amount DL.sub.n for effecting lens driving is found from the detected defocus amount DF.sub.n and lens driving is effected, proper focusing even to a moving object will always become possible at the end of lens driving.
The operation when the release operation has taken place during such automatic focus adjustment control will now be described with reference to FIGS. 3 and 4 of the accompanying drawings.
FIG. 3 shows a case where the release operation has taken place at a time t.sub.x1 under the situation that focus detection is started at a time t.sub.n and lens driving DL.sub.n is effected at a time t.sub.n, and lens driving is completed at a time t.sub.n+1. Here, the time from after the release operation has taken place until film exposure is actually effected, i.e., the so-called release time lag, is TR. Thus, in the figure, film exposure is effected at a time t.sub.x1 +TR. In the case where lens driving is stopped simultaneously with the taking-place of the release operation, the position l.sub.x1 of the image plane of the lens at the time t.sub.xl is the position l.sub.r1 of the image plane of the lens at the time t.sub.x1 +TR, and at this time, the image plane of the object is positioned at f.sub.r1 and therefore, the object image exposed on the film suffers from defocus of f.sub.r1 -l.sub.r1 =d.sub.x1, that is, an out-of-focus condition occurs.
In the case where lens driving is continued even if the release operation takes place, l.sub.n+1' is reached at a time t.sub.n+1, and the position of the image plane of the lens at the time t.sub.x1 +TR is l'.sub.r1, and an out-of-focus condition of f.sub.r1 -l'.sub.r1 =d'.sub.x1, though small in amount, still occurs.
FIG. 4 shows a case where the release operation has taken place during lens driving. In the case where as in the case of FIG. 3, lens driving is stopped simultaneously with the release operation, an out-of-focus condition of f.sub.r2 -l.sub.r2 =d.sub.x2 occurs, and in the case where lens driving is continued, an out-of-focus condition of f.sub.r2 -l'.sub.r2 =d'.sub.x2 occurs.
A description will now be given of a correcting method which takes a uniform release time lag into consideration. In this case, the time t.sub.n+1 may be considered to extend by an amount corresponding to the release time lag TR and therefore, the equation (10) is modified as follows: EQU DL.sub.n =a.sub.n .multidot.{(TM.sub.n-2 +TM.sub.n-1 +TM.sub.n +TR).sup.2 -(TM.sub.n-2 +TM.sub.n -1).sup.2 }+b.sub.n .multidot.(TM.sub.n +TR)+DF.sub.n ( 11)
FIG. 5 of the accompanying drawings shows the control of the above equation (11). A curve f'(t) indicated by a dot-and-dash line is the position of the image plane of the object which takes the uniform release time lag TR into consideration, and the lens may be controlled so as to be along this curve. Accordingly, the object in the view-finder is always out of focus by an amount corresponding to the release time lag. Assuming that, as in FIG. 3, the release operation has taken place at the time t.sub.x1, where lens driving is stopped, the position of the image plane of the lens is l.sub.r1 at the time t.sub.x1 +TR, and the actual position of the image plane of the object is f.sub.r1 and therefore, an out-of-focus condition of f.sub.r1 -l.sub.r1 =d.sub.x1 occurs. Where lens driving is continued, an out-of-focus condition of f.sub.r1 -l'.sub.r1 =d'.sub.x2 occurs. FIG. 6 of the accompanying drawings shows a case where the release operation has taken place during lens driving, and where lens driving is stopped simultaneously with the release operation, an out-of-focus condition of f.sub.r2 -l.sub.r2 =d.sub.x2 occurs, and where lens driving is continued, an out-of-focus condition of f.sub.r2 -l'.sub.r2 =d'.sub.x2 occurs.
As described above, again in the aforedescribed method which takes the release time lag into consideration, considerably good correction is possible although more or less an out-of-focus condition remains depending on the timing of release.
FIG. 7 of the accompanying drawings newly depicts the manner of the first and subsequent focus detecting operations in a case where the correcting system of FIG. 5 or 6 is applied. From the defocus amounts DF1, DF2 and DF3 and the lens driving amounts DL1 and DL2 obtained at the times t.sub.1, t.sub.2 and t.sub.3, a.sub.3 and b.sub.3 are determined by the use of the equations (8) and (9), and if lens driving is effected after DL3 is calculated from the equation (11), the lens reaches l.sub.4 at a time t.sub.4. When a release signal comes at this point of time, release takes place after TR and at this time, the image plane of the object is at f.sub.r4 and therefore coincides with the lens position l.sub.4, and a photograph which is in focus can be taken. If the release signal does not come, the aforedescribed focus detecting operation cycle is repeated, and the release positions after the fourth and fifth focus detecting operations and l.sub.5 and l.sub.6, respectively.
Now, in the above-described example, the position of the object is approximated by a quadratic function and therefore, the lens positions after l.sub.4 are accurately driven to desired positions (the dot-and-dash line in the figure), but it is after the third lens driving termination time t.sub.4 that this correction effect appears. Accordingly, even if the release signal comes before that, the correction will not be effective and a photograph which is out of focus will be taken, or to ensure an in-focus condition, release must be waited for until t.sub.4. So, to apply a correction a little earlier, the position of the object may be approximated by a linear function. This is shown in FIG. 8 of the accompanying drawings.
If the image planes of the object at the times t.sub.1 and t.sub.2 are f.sub.1 and f.sub.2, the linear functional equation passing through these two points is EQU pt+q=g(t) (12)
Representing p and q with l.sub.1 as the origin and by the use of DF1, DL1 and TM1, ##EQU4## EQU q=DF1. (14)
Consequently, the foreseen position of the image plane of the object which takes the release time lag into consideration is l.sub.r3, and the required lens driving amount DL is EQU DL=p(TM1+TM2+TR)+DF1-DL1. (15)
If the above-described operation is repeated, the positions in which lens driving is completed are l.sub.3, l.sub.4, l.sub.5, . . . , but these have errors relative to the desired position f'(t) (dot-and-dash line) which takes the release time lag into consideration. If the position of the object is thus approximated by a linear function, a considerable correction error occurs in a case where the position of the image plane of the object is not linear relative to time. That is, it will be seen that if the function applied when the position of the object is supposed is fixed to one kind, the time until correction begins to be effective (the correction time lag) and the correction accuracy are contrary to each other.
Also, according to the foreseeing method using the quadratic function represented by the equations (8), (9) and (10) described above with respect to FIG. 2, if the position of the image plane of the object changes in accordance with the quadratic function and the defocus amount detected at each time is accurate, proper correction is accomplished as described above, but if proper detection of the defocus amount fails to be accomplished due to a cause such as the low contrast of the object, the error will be enlarged and excessive correction will be made because the correction equation is the extrapolation of the quadratic function.
That is, when as shown in FIG. 16 of the accompanying drawings, relative to the actual positions f.sub.1, f.sub.2 and f.sub.3 of the image plane, the positions f.sub.1 ', f.sub.2 ' and f.sub.3 ' of the image plane are detected as the defocus amounts resulting from the focus detection (that is, when an error occurs to the defocus amount), the position of the image plane at the time t.sub.4 foreseen by the quadratic function on the basis of f.sub.1 ', f.sub.2 ' and f.sub.3 ' is l.sub.4, and if the lens is driven to this position, the out-of-focus situation represented by er in the figure occurs at the time t.sub.4.